The 3 rectangles represent 3 sides of the prism. Imagine folding on the line, and connecting the left side of the first rectangle to the right side of the third rectangle. It will make a triangle that has depth.
Looking that the top and bottom triangles attached to the third rectangle shows us that the top and bottom of this shape are triangles, which matches the three sides.
What we have is a 3D triangle, which we call a triangular prism
